Low density foam resin composite of high stiffness

ABSTRACT

The present invention relates to a method of producing low density  polydi resins having high yield strengths and the products thereof. In particular a polydiene resin, such as dihydroxyl terminated 1,2-polybutadiene is reacted at room temperature with an organic chain extender, such as hexamethylene -1,6-diisocyanate, and a graft comonomer such as N-vinyl-2-pyrrolidone in the presence of a peroxide free radical initiator, such as O,O&#39;-bis(t-butyl peroxy) diisopropylbenzene, whereby a elastomeric material having the peroxide and a graft comonomer dispersed therethrough is produced. At a subsequent period of time, the elastomeric material can be exposed to elevated temperatures whereupon the elastomer is cured to a low density resin having a high yield strength.

BACKGROUND OF THE INVENTION

This invention relates to polydiene resins and more particularly topolydiene resins having either urethane or epoxyl groups.

For a variety of applications (e.g. deep sea buoyancy structures,aviation structures, etc.) low density plastic resins having highcompressive yield strengths are needed. The state of the art teachesthat functionally terminated 1,2-polybutadiene resins, which are chainextended with a diisocyanate or diepoxide, may exhibit a low density anda remarkable ultimate strength exceeding that of many other highstrength plastic materials. However, these resins lack high yieldstrengths and high elastic moduli.

For example, resin composite obtained from dihydroxyl 1,2-polybutadieneand toluene-2,4-diisocyanate had, after ring closure and/or crosslinkingwith a peroxide, a density of 1.04 and an ultimate compressive strengthof 38,400 psi but a yield strength of only 5,500 psi and a compressivemodulus of 225,000 psi. By comparison, a conventional epoxy resin had adensity of 1.24, an ultimate compressive strength of 21,000 psi, a yieldstrength of 18,000 psi, and a compressive modulus of 500,000 psi.However, because conventional epoxy resins have high densities, they areless desirable for aviation or deep sea buoyancy structures than the lowdensity polydiene resins. Therefore, it would be desirable to find novellow density polydiene resins having higher yield strengths than theprior art low density polydiene resins.

SUMMARY OF THE INVENTION

Accordingly, one object of this invention is to provide novel lowdensity polydiene resins.

Another object of this invention is to provide low density thermosettingpolyolefinic resins having higher yield strengths.

A further object of this invention is to provide low density, high yieldstrength, thermosetting resins having viscosity and cure characteristicssuitable for the fabrication of castable plastic materials under theconditions of vacuum potting.

A still further object of this invention is to provide a method ofmanufacturing low density polydiene resins having higher yieldstrengths.

These and other objects of this invention are accomplished by providinga resin which is produced by reacting (A) a difunctional polydieneprepolymer having (1) two terminal functional groups selected from thegroup consisting of hydroxyl and carboxyl and (2) a predominant amountof vinyl groups on alternate carbon atoms of the polydiene backbone with(B) a chain extender, provided that when the terminal functional groupson the polydiene are hydroxyl groups, the chain extender is adiisocyanate, but when the functional groups on the polydiene arecarboxyl groups, the chain extender is a diepoxide, at ambienttemperature in the presence of (C) a graft comonomer selected from thegroup consisting of N-vinyl-2-pyrrolidone, N-vinyl-3-pyrrolidone,N-vinylcarbazole, N-vinylimidazole, N-vinylpyridine, coumarone, indene,and mixtures thereof, and (D) a peroxide free radical initiator toproduce the reaction product. This reaction product can then be cured atelevated temperatures to produce the final polydiene resin.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The backbone of the resin of the present invention is formed either (1)by reacting a dihydroxyl polydiene prepolymer with a diisocyanate as achain extender to form a polyurethane polydiene resin or (2) by reactinga dicarboxyl polydiene with a diepoxide to form an epoxy polydieneresin. Because of their greater yield strengths, the polyurethanepolydiene resins are preferred over the epoxy polydiene resins.

The dihydroxyl or dicarboxyl terminated polydiene prepolymers used inthe present invention should have a high residual vinyl or alkenylcontent. Polydienes derived from isoprene, dimethylbutadiene,methylpentadiene, or butadiene are suitable, with dicarboxyl1,2-polybutadiene and dihydroxyl 1,2-polybutadiene being preferred anddihydroxyl 1,2-polybutadiene being more preferred. Dihydroxyl anddicarboxyl polydiene prepolymers having molecular weights of from about600 to about 3000 are preferred. If the molecular weight exceeds 3000,the prepolymers become too viscous for proper casting and pottingoperations.

The chain extenders for the dihydroxyl polybutadiene prepolymers may bearomatic as represented by toluene-2,4-diisocyanate ornaphthalene-1,5-diisocyanate. However, the non-aromatic diisocyanates(e.g. hexamethylene-1,6-diisocyanate,2,2,4-trimethylhexamethylene-1,6-diisocyanate, isophorone diisocyanate,dicyclohexylmethane diisocyanate) are preferred because they undergourethane and allophanate formation rather slowly thus displaying lowviscosities during the casting and vacuum potting operation. Isophoronediisocyanate is the more preferred chain extender.

The key feature of this invention is the grafting of a cylic monomerselected from the group consisting of N-vinyl-2-pyrrolidone, N-vinyl-3-pyrrolidone, N-vinylcarbazole, N-vinylimidazole, N-vinylpyridine,coumarone, indene and mixtures thereof onto the polymer backbone. Thepreferred graft monomers are N-vinyl-2-pyrrolidone,N-vinyl-3-pyrrolidone, and mixtures thereof.

Although the free radical initiator used is not a critical limitation,0,0'-bis(t-butylperoxy) diisopropylbenzene works well as the catalyst.Preferably about 2 parts by weight of the free radical initiator shouldbe used for every 100 parts by weight of the dihydroxyl or dicarboxylpolydiene prepolymer.

The preferred range of the molar ratio of aliphatic diisocyanate todihydroxyl polydiene prepolymer is from about 0.75:1 to about 1.30:1,with from 0.95:1 to 1.10:1 being the more preferred range. For instance,preferably from 12 to 20 and more preferably from 15 to 18 parts byweight of isophorene diisocyanate chain extender (MW=222) should be usedwith every 100 parts by weight of dihydroxyl 1,2-polybutadieneprepolymer (MW = 1420).

If aromatic diisocyanates are reacted with the dihydroxyl polydieneprepolymer, an excess of the aromatic diisocyanate should be used forproper viscosity control. Thus, the preferred range of the molar ratioof aromatic diisocyanate to dihydroxyl polydiene prepolymer is fromabout 0.95:1 to about 1.65:1, with from 1.20:1 to 1.50:1 being the morepreferred range. For example, preferably from about 12 to about 20 andmore preferably from 15 to 18 parts by weight toluene diisocyanate chainextender (MW = 174) should be used for every 100 by weight of thedihydroxyl 1,2-polybutadiene prepolymer (MW = 1420) used.

The preferred range of the molar ratio of the cyclic graft comonomerused in this invention to the dihydroxyl or dicarboxyl polydieneprepolymer is from about 0.60:1 to about 6.40:1 with from 4.45:1 to5.75:1 being the more preferred range. For instance, from about 5 toabout 50, and more preferably from 35 to 45 parts by weight ofN-vinyl-2-pyrrolidone (MW = 111) should be used for every 100 parts byweight of dihydroxyl 1,2-polybutadiene prepolymer (MW = 1420). Theamount of N-vinyl-2-pyrrolidone which can be used for a given weight ofprepolymer is limited by the hygroscopic properties of the vinylpyrrolidone.

These cyclic graft comonomers are grafted onto an already existingpolydiene polymer backbone and therefore are to be distinguished frompolymers where the cyclic comonomers are part of the backbone itself.

The difunctional polydiene prepolymer, chain extender, cyclic graftcomonomer, and free radical initiator are all mixed together and allowedto react at ambient temperature for about 1 hour. Later the reactionmixture may be poured into casts or structural spaces and cured atelevated temperatures.

Moreover, by embedding glass or phenolic microspheres into the reactionmixture before the curing step at elevated temperatures, syntacticbuoyancy foams having superior strength and buoyancy characteristics areobtained. Note that before the curing step at elevated temperatures, thereaction mixture contains some unreacted isocyanate groups which have aninherent reactivity with silanol (or other metal hydroxide) groupslocated in the surface of the glass microspheres. The pyrrolidonemoieties contribute further to the adhesiveness of the resin matrix. Andfinally, adhesion promotors derived from silyperoxides (e.g.tris(tert-butylperoxy) vinyl silane) drastically increase bond strengthand they reduce water absorption of the glass resin interface.

The following examples serve to illustrate the present inventionwithout, however, limiting the same thereto:

EXAMPLE 1

To 100 parts by weight dihydroxyl 1,2-polybutadiene having theproperties:

    Molecular Weight    1420                                                      Vinyl Content,%     91.5                                                      Hydroxy Content, Meg/gr                                                                           1.19                                                      Viscosity, poises at 45°C                                                                  55                                                    

there was added a mixture of 15 parts by weight isophorone diisocyanate,40 parts by weight N-vinyl-2-pyrrolidone and 4 parts by weight0,0'-bis(t-butylperoxy) diisopropylbenzene (commercially known as"Vul-Cup R"). After stirring until homogenous, the resulting reactionproduct maintained over a period of one hour the followingtemperature-viscosity relationship:

    TEMPERATURE (°C)                                                                         VISCOSITY (cp)                                              ______________________________________                                        24                550                                                         50                220                                                         80                110                                                         110                50-100                                                     ______________________________________                                    

The product was poured into a mold and cured consecutively 48 hours at90°C plus 24 hours each at 100°C, 110°C and 120°-200°C.

The cured castings had the following strength characteristics:

    Yield Strength (0.2% offset), psi                                                                  11800                                                    Ultimate Strength, psi                                                                             27500                                                    Compressive Modulus, psi                                                                           450000                                                   Density (gr/cc)      1.04                                                 

EXAMPLE 2

Example 1 was repeated omitting the N-vinyl-2-pyrrolidone. The viscosityrose from 350 to 1250 centipoises at 90°C over a period of 1 hour andthe following strength characteristics of the cured resin weredetermined:

    Yield Strength (0.2% offset), psi                                                                  7400                                                     Ultimate Strength, psi                                                                             36000                                                    Compressive Modulus, psi                                                                           367000                                                   Density (gr/cc)      1.04                                                 

EXAMPLE 3

To successive portions of 200 parts by weight dihydroxyl1,2-polybutadiene having properties as described in Example 1, therewere added 36 parts by weight toluene-2,4-diisocyanate and subsequently10, 20, and 40 parts by weight N-vinyl-2-pyrrolidone. Finally, 4 partsby weight 0,0'-bis(t-butylperoxy) diisopropylbenzene was stirred intothe mixture.

Castings made were cured consecutively 24 hours each at 90°- 115°C,105°- 120°C, and 120°-140°C.

The following table summarizes the compositions (in parts by weight) andthe strength characteristics of the cured castings:

                    A    B    C    D                                              __________________________________________________________________________    dihydroxyl 1,2-polybutadiene                                                                  200.0                                                                              200.0                                                                              200.0                                                                              200.0                                          Toluene-2,4,diisocyanate                                                                      36.0 36.0 36.0 36.0                                           N-vinyl-2-pyrrolidone                                                                         0.0  10.0 20.0 40.0                                           Vul-Cup R       4.0  4.0  4.0  4.0                                            Compressive Yield Strength,                                                                   8100 9781 9970 11000                                          psi (0.2% offset)                                                             Ultimate Strength, psi                                                                        42000                                                                              28000                                                                              21000                                                                              25800                                          Compressive Modulus, psi                                                                      328000                                                                             331000                                                                             345000                                                                             385000                                         Density (gr/cc) 1.00 1.03 1.03 1.04                                           __________________________________________________________________________

EXAMPLE 4

Mixtures of dihydroxyl 1,2-polybutadiene, isophoronediisocyanate,N-vinyl-2-pyrrolidone, trimethylolpropane trimethacrylate and0,0'-bis(t-butylperoxy) diisopropylbenzene were prepared from thefollowing aliquot parts:

                    A    B    C    D    E                                         __________________________________________________________________________    dihydroxyl 1,2-polybutadiene                                                                  200.0                                                                              200.0                                                                              200.0                                                                               200.0                                                                             200.0                                     Isophoronediisocyanate                                                                        30.0 30.0 30.0 30.0 30.0                                      N-vinyl-2-pyrrolidone                                                                         20.0 40.0 40.0 20.0 20.0                                      Trimethlolpropane                                                             Trimethacrylate 0.0  0.0  20.0 10.0 10.0                                      Vul-Cup R       4.0  4.0  4.0  4.0  4.0                                       __________________________________________________________________________

After mixing, casting and curing as described in Example 1, thefollowing strength data were obtained:

                  A    B    C    D    E                                           __________________________________________________________________________    Yield Strength                                                                              7800 9700 10350                                                                              8400 9150                                        (0.2% offset), psi                                                            Ultimate Strength, psi                                                                      22000                                                                              17600                                                                              28400                                                                              31500                                                                              31000                                       Compressive Modulus, psi                                                                    339000                                                                             378000                                                                             437000                                                                             387000                                                                             400000                                      Density (g/cc)                                                                              1.04 1.04 1.04 1.04 1.04                                        __________________________________________________________________________

EXAMPLE 5

A syntactic foam buoyancy material was prepared by the maximum randompacking of B-35D hollow glass microspheres (3M Company) in a mixture ofa low density prepolymer containing the modifiers. The basic resinmatrix consisted of 325 parts (by weight) dihydroxy 1,2-polybutadiene,49 parts isophorone diisocyanate and 13 parts 0,0'-bis(tert-butylperoxy) diisopropylbenzene (commercially known as "Vul-CupR") used as vulcanizing agent. Modification for increased stiffness andinterfacial bonding comprised 130 parts vinylpyrrolidone, 65 partstrimethylolpropane trimethacrylate and 13 parts (tert-butylperoxy) vinylsilane. After deaeration in a vacuum potter, the obtained glass-resincomposite was submitted to the following cure cycle: 48 hours at 90°C,48 hours at 100°-110°C and 24 hours at 120°-200°C. The syntactic foamthus obtained had physical characteristics as follows:

    Specific Gravity       0.567 (35.3 pcf)                                       Compressive Hydrostatic Strength, psi                                                                16,200                                                 Ultimate Compressive Strength, psi                                                                   11,500                                                 0.2% Offset Yield Strength, psi                                                                      11,100                                                 Compressive Modulus, psi                                                                             427,000                                                Water Absorption after 168 hours)                                              at 9000 psi, percent  3.1                                                

EXAMPLE 6

A syntactic foam buoyancy material was prepared by the maximum randompacking of B-35D hollow glass microspheres (3M Company) in a mixture ofa low density resin prepolymer containing the modifiers. The basic resinmatrix consisted of 325 parts (by weight) dihydroxyl 1,2-polybutadiene,49 parts isophorone diisocyanate and 13 parts 0,0'-bis(tert-butylperoxy)diisopropylbenzene. After deareration in a vacuum potter, the obtainedglass-resin composite was submitted to the following cure cycle: 48hours at 90°C, then 48 hours at from 100° to 110°C, and finally 24 hoursat from 120° to 200°C. The syntactic foam thus obtained had physicalcharacteristics as follows:

    Specific Gravity       0.552 (34.4 pcf)                                       Compressive Hydrostatic Strength, psi                                                                14,900                                                 Ultimate Compressive Strength, psi                                                                   10,900                                                 0.2% Offset Yield Strength, psi                                                                       9,800                                                 Compressive Modulus, psi                                                                             334,000                                                Water Absorption (after 168 hours                                              at 900 psi), percent  6.1                                                

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. A process for producing a syntactic foamcomprising: (I) reacting (A) a dihydroxy polydiene having (1) twoterminal hydroxy groups and (2) a predominate amount of vinyl groups onalternate carbon atoms of the polydiene backbone with (B) an organicchain extender which is a diisocyanate in the presence of (C) a graftcomonomer selected from the group consisting of N-vinyl-2-pyrrolidone,N-vinyl-3-pyrrolidone, N-vinylcarbazole, N-vinylimidazole,N-vinyl-pyridine, coumarone, indene and mixtures thereof, (D) a peroxidefree radical initiator, and (E) a tris(alkylperoxy) vinyl silane toproduce an elastomeric material having the peroxides,, thetris(alkylproxy) vinyl silane, and the graft comonomer dispersedtherethough substantially unreacted, (II) packing microspheres into theelastomeric material wherein the microspheres are selected from thegroup consisting of hollow glass microspheres, hollow phenolicmicrospheres and mixtures thereof, and then (III) curing the elastomerat elevated temperatures to produce a firm resinous material containingthe microspheres.
 2. The process of claim 1 wherein thetris(alkylperoxy) vinyl silane is tris(tert-butylperoxy) vinyl silane.3. The product of the process of clain
 1. 4. The product of the processof claim 2.